mkfs_minix: stop using lots of bss/data.

data -3500 bytes, code -300 bytes
keep_data_small: expand/fix

docs/keep_data_small.txt

@@ -1,4 +1,4 @@
-	Keeping data small
+		Keeping data small
 
 When many applets are compiled into busybox, all rw data and
 bss for each applet are concatenated. Including those from libc,
@@ -10,6 +10,11 @@ On nommu it's probably bites the most, actually using real
 RAM for rwdata and bss. On i386, bss is lazily allocated
 by COWed zero pages. Not sure about rwdata - also COW?
 
+In order to keep bbox NOMMU and small-mem systems friendly
+we should avoid large global data in our applets, and should
+minimize usage of libc functions which implicitly use
+such structures in libc.
+
 Small experiment measures "parasitic" bbox memory consumption.
 Here we start 1000 "busybox sleep 10" in parallel.
 bbox binary is practically allyesconfig static one,
@@ -34,14 +39,10 @@ bash-3.2# nmeter '%t %c %b %m %p %[pn]'
 23:17:43 ..........    0    0 168M    0  147
 
 This requires 55M of memory. Thus 1 trivial busybox applet
-takes 55k of userspace memory (nmeter doesn't account for kernel-side
-allocations). Definitely can be improved.
+takes 55k of memory.
 
-Thus we should avoid large global data in our applets,
-and should minimize usage of libc functions which implicitly use
-such structures in libc.
 
-	Example 1
+		Example 1
 
 One example how to reduce global data usage is in
 archival/libunarchive/decompress_unzip.c:
@@ -57,12 +58,15 @@ archival/libunarchive/decompress_unzip.c:
 #define STATE_IN_BSS 0
 #define STATE_IN_MALLOC 1
 
+(see the rest of the file to get the idea)
+
 This example completely eliminates globals in that module.
 Required memory is allocated in inflate_gunzip() [its main module]
 and then passed down to all subroutines which need to access 'globals'
 as a parameter.
 
-	Example 2
+
+		Example 2
 
 In case you don't want to pass this additional parameter everywhere,
 take a look at archival/gzip.c. Here all global data is replaced by
@@ -70,7 +74,7 @@ single global pointer (ptr_to_globals) to allocated storage.
 
 In order to not duplicate ptr_to_globals in every applet, you can
 reuse single common one. It is defined in libbb/messages.c
-as struct globals *ptr_to_globals, but the struct globals is
+as struct globals *const ptr_to_globals, but the struct globals is
 NOT defined in libbb.h. You first define your own struct:
 
 struct globals { int a; char buf[1000]; };
@@ -79,13 +83,45 @@ and then declare that ptr_to_globals is a pointer to it:
 
 #define G (*ptr_to_globals)
 
-Linker magic ensures that these two merge into single pointer object.
-Now initialize it in <applet>_main():
+ptr_to_globals is declared as constant pointer.
+This helps gcc understand that it won't change, resulting in noticeably
+smaller code. In order to assign it, use PTR_TO_GLOBALS macro:
 
-	ptr_to_globals = xzalloc(sizeof(G));
+	PTR_TO_GLOBALS = xzalloc(sizeof(G));
 
-and you can reference "globals" by G.a, G.buf and so on, in any function.
+Typically it is done in <applet>_main().
 
-The drawback is that now you have to initialize it by hand. xzalloc()
-can be helpful in clearing allocated storage to 0, but anything more
-must be done by hand.
+Now you can reference "globals" by G.a, G.buf and so on, in any function.
+
+
+		bb_common_bufsiz1
+
+There is one big common buffer in bss - bb_common_bufsiz1. It is a much
+earlier mechanism to reduce bss usage. Each applet can use it for
+its needs. Library functions are prohibited from using it.
+
+'G.' trick can be done using bb_common_bufsiz1 instead of malloced buffer:
+
+#define G (*(struct globals*)&bb_common_bufsiz1)
+
+Be careful, though, and use it only if
+sizeof(struct globals) <= sizeof(bb_common_bufsiz1).
+
+
+		Drawbacks
+
+You have to initialize it by hand. xzalloc() can be helpful in clearing
+allocated storage to 0, but anything more must be done by hand.
+
+All global variables are prefixed by 'G.' now. If this makes code
+less readable, use #defines:
+
+#define dev_fd (G.dev_fd)
+#define sector (G.sector)
+
+
+		Word of caution
+
+If applet doesn't use much of global data, converting it to using
+one of above methods is not worth resulting code obfuscation.
+If you have less that ~300 bytes of global data - don't bother.